Data is one of the most important asset of an organization especially as organizations rely more and more on data processing systems for their daily operations. Any loss of data or even loss of access to the data is therefore potentially very costly. For example, an hour of down time for a system handling brokerage operations has been estimated to cost eight million dollars. Current methods for preventing data loss include using redundant arrays of disk such as RAID. RAID protection alone is, however, not sufficient or cost effective at preventing data loss, especially in view of the following trends:                More and more disks are being grouped into an array, thereby increasing the chances of having data loss situations (e.g., multiple failures within an array).        Disks are growing in capacity faster than they are improving in data rate. As a result, the time to rebuild a disk is increasing over time, and this lengthens the window during which the array could be vulnerable to data loss.        Disk vendors are continuing to push areal density aggressively. Historically, this has caused a reduction in disk reliability which we expect will continue in the future.        System vendors are increasingly interested in building cost-effective storage systems with low-end desktop disks, which are generally regarded as being less reliable.        Widespread deployment of techniques like virtualization, which can spread a host LUN across many disk arrays, increase the impact to the user of a data loss situation because many more host LUNs could be impacted.        
RAID protection is typically augmented by periodically copying the data onto a secondary system such as a tape library in a process referred to as backing up. When the primary copy of the data fails, the data can be retrieved from the secondary system through a process called restore. However, the process of recovering data from the back-up copy tends to take at least several hours, even if the failure affects only a small number of blocks. This is because the disk array and the back-up system are not integrated so that a lot of data has to be unnecessarily retrieved from the back-up system. For instance, the minimum unit of recovery from the back-up system is typically a LUN or at least a database table. The result is that critical data is not available for long periods of time. Moreover, the data written after the last back up cannot be recovered from the back up, and is likely to be lost. In certain database environments where the log of all transactions is carefully kept, the database can be brought up to date by replaying the log. Doing so, however, requires a lot of operator involvement, is very time consuming and uses expensive host MIPS.